DC offset, which is often created at the amplification stage, is an undesired component in the analog and RF implementation of wireless communication. A DC offset can either corrupt the signals or saturate the following stage. The DC offset is particularly large after a down-conversion mixer as the down-conversion method translates the radio signal directly to baseband, in which the majority of the gain and filtering are performed in a frequency band from DC to the signal bandwidth. The DC offset in the signal path is amplified throughout the process and the amplified DC offset can further degrade the dynamic range of the circuit. In addition, the DC offset is created when some of the on-channel local oscillator (LO) signal leaks to input port of the mixer and is down-converted. The LO self-mixing effect caused by the LO signal leakage will further aggravate the DC offset problem. If the DC offset is not properly suppressed or cancelled, the sensitivity of the receiver using the down-conversion mixer will suffer greatly.
Various DC offset cancellation schemes have been developed to overcome this problem in the recent years. For examples, the AC-coupling method can be used to solve the problem for applications in which no useful information content in included in the DC signal. However, the large RC area occupation and the long settling time of the AC-coupling method will add an extra problem in the low-cost and high data rate transceiver circuits.
Another DC offset cancellation approach is to use a high-gain differential amplifier and a negative-feedback method. As shown in FIG. 1, a high-gain differential amplifier 108 is used to detect the output DC offset of a circuit 102, and a negative-feedback loop is used to force the DC offset down, or cancel the DC offset entirely This approach, commonly used in many design, uses two low-pass filters (LPF) 104, 106 to realize the differential DC feedback. Without the two LPFs, the AC signal will also be detected and cancelled by the feedback loop that cancels the DC offset. The conventional way to implement the low-pass filters 104, 106 can be either on-chip or off-chip. The former is problematic due to the high RC value, which results in occupying a large chip area, while the latter requires the consumption of two chip pins. It is, therefore, necessary to develop a new approach for reducing the use of the LPF.